18% grey in the metering principle of SLR camera

I have been struggling with this problem for a long time. I talk about my own views by consulting various materials and combining my own understanding.

Generally speaking, the reflectivity of an object is about 18% on average, so when manufacturing a camera, the reflectivity of camera metering is set to 18%, that is to say, whether you are metering light against snow or something very dark, it will default to 18% gray level, so the next point is coming.

For example, shooting a snow scene, why is the picture gray? That is, the default luminosity of the camera is 18%, and the default reflectivity of white is 18%, so it is gray. If you want to get the correct exposure, you must increase the exposure increment, or you can measure the light at the gray card first, so as to restore the white color. Similarly, the same is true for shooting black objects to reduce exposure. So, why is it sometimes overexposed? For example, when shooting portraits in cloudless weather in Wan Li, people usually aim at their faces to measure light. In a certain weather, the reflectivity of human face is lower than 18%, but the camera regards it as 18%. In this way, the original white sky will lose its details and become pale, so it can accurately measure the gray part of the sky (the same is true for gray cards).

This is also my own understanding. Please forgive me if it is biased.